This present invention relates to ultrasonic monitoring and intruder detection, particularly but not exclusively, for a vehicle interior.
A number of ultrasonic monitoring processes are known, as disclosed in, for example, German Patent No. DE 195 21 511, which relates to monitoring an interior compartment of a vehicle. The processes involve transmission of an ultrasonic pulse, receiving a resultant echo signal and comparing the echo signal envelope with a presettable reference profile or a preceding echo signal envelope. Intrusion may be assumed if the comparison returns a difference exceeding a threshold level. The interval between pulses is greater than the duration of the received echo signal such that each echo signal envelope is sequentially received without any overlap, to provide a representation of the entire vehicle interior.
In accordance with the present invention, there is provided an ultrasonic monitoring method in which a sequence of ultrasonic pulses is transmitted into a space, and a resultant echo signal is received. The space is monitored for intrusion by comparing the resultant echo signal received in a first window period with the resultant echo signal received is a second window period. The pulses are separated by a time interval such that echoes generated from the pulses superimposed to form the resultant signal.
Preferably, the echo signal received in the first window period is stored for real time comparison with the echo signal received in the second window period.
Preferably, the window periods are synchronised with the pulses.
Preferably, the method includes transmitting the sequence of pulses in a first operational period. The transmission of the pulses is terminated until a second operational period is commenced if the comparison returns a result below a threshold level, and the transmission of the pulses is continued as an intrusion sequence if the comparison returns a result above a threshold level.
In another aspect of the present invention, there is provided an ultrasonic monitoring system for use in the above described method. The system includes a transmitter, a receiver, and a microprocessor arranged to transmit a sequence of ultrasonic pulses from the transmitter at a predetermined interval and a receive and compare a resultant echo signal over a plurality of window periods. The interval is such that echoes generated from the pulses are superimposed to form the resultant signal.
Preferably, the microprocessor is adapted to terminate the sequence of pulses for the remainder of a first operational period and adapted to retransmit the sequence in a second operational period if the comparison returns a result below a threshold level. An intrusion sequence is actuated if the comparison returns a result above the threshold level.